[FIX] Use specific exception types in date_features

[K2alyan]

Summary

• Replace raise Exception(...) with raise ValueError(...) when a user passes an unsupported country code to CountryHolidays
• Replace raise Exception(...) with raise ImportError(...) from e when the optional holidays package is not installed, preserving the original exception chain
• Add a test test_country_holidays_invalid_country that verifies the ValueError path

Motivation

Generic Exception makes it hard for callers to handle errors specifically. ValueError is the right type for invalid input (a bad country code), and ImportError is the right type for a missing dependency. This also chains the original ModuleNotFoundError so the traceback retains the root cause.

Test plan

• Run pytest nixtla_tests/date_features/ — all three tests should pass
• The new test test_country_holidays_invalid_country verifies the ValueError is raised when an invalid country code is provided

[CLAassistant]

All committers have signed the CLA.